1. Field of the Invention
The present invention relates to a cathode electrode, a method for manufacturing the same, and to a lithium battery containing the same. More particularly, the present invention relates to a cathode electrode that maintains its structure when sulfur contained in a cathode active material is dissolved in an electrolytic solution during discharge, to a method for manufacturing the same, and to a lithium battery containing the same.
2. Description of the Related Art
Theoretically, a sulfur monomer has a very high electrochemical capacity of 1680 mAh/g and has voltage plateau region of approximately 2 V with respect to lithium metal. A cathode electrode using sulfur has a higher capacity than a conventional cathode for a lithium secondary battery. Thus, intensive research into cathode electrodes using sulfur is being carried out to be used for next generation secondary batteries.
It is known that sulfur cathode active materials undergo various electrochemical reactions as follows. However, the reaction capacity of sulfur that can be actually used for a lithium secondary battery is only a half a theoretical capacity, that is, approximately 840 mAh/g, due to irreversibility of some polysulfides.                2Li+S8 (solid)⇄Li2S8 (solution)        2Li+Li2S8 (solution)⇄2Li2S4 (solution)        2Li+Li2S4 (solution)⇄2Li2S2 (solution)        2Li+Li2S2 (solution)⇄2Li2S8 (solid precipitate)        
Sulfur and sulfur-containing organic compounds, which are cathode active materials, have no or very low electrical conductivity. Thus, in order to cause an electrochemical reaction, it is necessary for the cathode active material to contact the surface of a conductive reaction site. Also, in order to smoothly provide electrochemical reaction sites, it is necessary to establish a sufficient reaction surface area for reaction through the use of large amounts of conductive materials. In particular, the use of sulfur monomers as a cathode active material cause phase transition from solid sulfur monomer (S8) to polysulfides as a discharge product Li2SX expected to be dissolved in electrolytic solution during an initial discharge stage.
Known cathode electrodes using sulfur are disclosed in U.S. Pat. Nos. 5,523,179 and 5,582,623, in which sulfur as an active material and carbon powder as a conductive material are simply mixed, occupying separate spaces in a cathode active material layer, so that the space occupied by solid sulfur is collapsed due to dissolution of sulfur in electrolytic solution during phase transition from sulfur to polysulfide at charging and discharging stages. This adversely affects battery characteristics, especially charge and discharge performance and life characteristic of a lithium battery.